The present invention relates to a golf ball having a superior impact resilience (flying performance) and shot feeling (realize a soft feeling), a golf ball cover material composition capable of obtaining the golf ball having the superior impact resilience and durability and a good appearance, and a method of manufacturing the composition.
There are two types of golf balls: a two piece golf ball in which a core is covered with a cover material; and a multi-layer structure golf ball having three or more layers in which appropriate intermediate layer or layers are placed between the core and the cover.
In a golf ball, a core having a rubber composition is covered by a cover material such as ionomer resin (ionic ethylene copolymer) having superior impact resistance and durability. More particularly, since a two piece golf ball has a longer flight distance and greater durability, it is preferred by most golfers. However, a two piece golf ball is inferior to a thread-wound golf ball in shot feeling and spin performance, because its feeling is harder at the time of a shot.
Golf balls are proposed in the prior art (Japanese Patent Application Laid-open No. 4-109971, No. 5-123422 or the like) which make the shot feeling soft by softening the two piece golf ball like a golf ball that uses a soft core and a soft cover material, or a golf ball in which the hardness of the core is different from that of the cover material or a golf ball in which the thickness of the cover is adjusted and so on. However, although these golf balls are superior in the spin performance and the shot feeling, their flying performance is insufficient.
The impact resilience and shot feeling of the golf ball are also inconsistent with each other, and thus it is desired that a golf ball adapted to appropriately balance these properties is developed. In order to improve the feeling and the spin performance, golf balls have been proposed (Japanese Patent Application Laid-open No. 6-319830) in which the difference in the hardness between the core and the ball is reduced and the hardness of the cover and the thickness of the cover are adjusted. However, although this golf ball is superior in the shot feeling, its impact resilience is insufficient.
On the other hand, in order to make the shot feeling of the golf ball softer, constitution of a golf ball in which the core is covered with the cover material of the ionomer resin and diene rubber has been recently proposed (Japanese Patent Application Laid-open No. 6-79017). However, if the cover material of the ionomer resin and the diene rubber is used, color of the cover changes during or after manufacturing the golf ball, and thus the commercial value of the golf ball is disadvantageously reduced. Furthermore, when the ionomer resin and the diene rubber are kneaded for manufacturing the cover material made of the ionomer resin and the diene rubber, blocking of the diene rubber often occurs before introducing the diene rubber into a screw extruder. When such blocking occurs, it becomes difficult to supply the material of the ionomer resin and the diene rubber to the screw extruder, and, as a result, the productivity is disadvantageously reduced.
A first object of the present invention is to provide a golf ball wherein both of the impact resilience and shot feeling are improved.
A second object of the present invention is to provide a golf ball cover material composition capable of obtaining a golf ball having superior impact resilience and durability and a good appearance, and a method of manufacturing the composition.
(1) In order to achieve the first object, the present invention provides a golf ball whose core is covered with a cover, wherein a ratio of the flexural rigidity value of the golf ball to the flexural rigidity value of the core is 1.8 or more and less than 3.0 and a thickness of the cover is 1.7 to 2.6 mm.
For a golf ball, an increase in the of hardness of a ball allows the impact resilience to be increased. But on the other hand, the harder ball results in the harder feeling, and consequently the shot feeling becomes worse. On the contrary, although the softer ball makes the shot feeling better, the impact resilience becomes worse and, as a consequence, the ball does not fly longer.
Therefore, according to the present invention, taking into account a balance between the hardness and the softness of the ball, the ratio of flexural rigidity value is determined and the thickness of the cover is determined as described above, whereby both of the impact resilience and the shot feeling can be improved.
(2) In order to achieve the first object, the present invention also provides a golf ball whose core is covered with a cover, wherein a ratio of the compression value of the golf ball to the compression value of the core is 1.2 or more and less than 1.5 and a hardness of the cover is Shore D hardness of 62 or less.
The compression value and the hardness of the cover are determined in this way, and thus the first object can be achieved. Here, the compression value of the golf ball means a compressive strength (kgf) when the golf ball is deformed by 2.54 mm ({fraction (1/10)} inch). Also, the compression value of the core means the compressive strength (kgf) when the core is deformed by 2.54 mm ({fraction (1/10)} inch).
(3) In order to achieve the first object, the present invention further provides a golf ball whose core is covered with a cover, wherein the core is constituted of a rubber composition which is compounded of 100 parts by weight of matrix rubber containing polybutadiene rubber having 40% or more of cis-1,4-structure and 20 to 40 parts by weight of unsaturated carboxylic acid metal salt having an average particle size of 5.0 um or less, and the cover is constituted of a resin composition which is composed of ionomer resin and diene rubber dispersed in the ionomer resin and which has Shore D hardness of 65 or less, preferably 62 or less.
Compared to the prior art, the finely-powdered unsaturated carboxylic acid metal salt having average particle size of 5.0 m or less is blended with the matrix rubber, whereby it is more finely dispersed in the matrix rubber. In addition, since this unsaturated carboxylic acid metal salt acts as a co-crosslinking agent, it is finely dispersed whereby matrix rubber molecules are more tightly bonded to unsaturated carboxylic acid metal salt molecules, and thus a core composition having proper hardness and superior impact resilience is obtained.
Moreover, since the cover is constituted of the resin composition having Shore D hardness of 65 or less, preferably 62 or less, wherein the diene rubber is dispersed in the ionomer resin, the cover also has the appropriate hardness.
It is thus possible to provide a solid golf ball having superior flying performance, shot feeling and spin performance.
Here, xe2x80x9caverage particle sizexe2x80x9d means the particle size measured using a typical particle size analysis (using a centrifugal settlement system).
(4) In order to achieve the second object, a golf ball cover material composition of the present invention is characterized by that the composition has 50 to 90 parts by weight of ionomer resin, 10 to 50 parts by weight of diene rubber and 0.1 to 5 parts by weight of non-staining antioxidant, the diene rubber is dispersed in the ionomer resin, and the dispersed diene rubber has a particle diameter of 5.0 xcexcm or less. Furthermore, in a method of manufacturing this golf ball cover material composition, 50 to 90 parts by weight of ionomer resin, 10 to 50 parts by weight of diene rubber and 0.1 to 5 parts by weight of non-staining antioxidant are kneaded at a temperature between 130 to 260xc2x0 C., preferably 150 to 260xc2x0 C., with the air shut out.
Since this combination of the ionomer resin, the diene rubber and the non-staining antioxidant is used, the second object can be achieved. According to the present invention, since the color of the cover does not change, if a whitening agent such as titanium dioxide is added to the cover material white appearance is not marred even if white paint is not applied to the cover surface. Therefore, only the application of clear paint for protecting the cover is sufficient to paint-surface-finish the golf ball. Thus, the process is easier than the successive application of primer paint, white enamel paint and clear paint like the prior art.
(5) In order to achieve the second object, the present invention provides a method of manufacturing a golf ball cover material composition constituted by kneading ionomer resin and diene rubber and dispersing the diene rubber in the ionomer resin, wherein the ionomer resin and the diene rubber are previously mixed, at a weight ratio (ionomer resin/diene rubber) ranging from 9/1 to 5/5, at such a temperature at which the diene rubber is not crosslinked, and then the resultant mixture is kneaded at such a temperature at which the diene rubber is crosslinked.
In this way, the ionomer resin and the diene rubber are previously mixed at such a temperature at which the two do not crosslink, and then they are kneaded at such a temperature at which they crosslink. That is, two steps are carried out. Thus, since it is possible to prevent an inability to supply the diene rubber to a screw extruder due to the blocking of the diene rubber before kneading, the productivity can be improved. Since this prevention of the blocking also allows the ionomer resin and the diene rubber to be uniformly kneaded, it is possible to obtain a golf ball cover material composition constituting a golf ball having superior impact resilience and durability and good appearance.
In order to achieve the second object, the present invention provides a method of manufacturing a golf ball cover material composition constituted by kneading ionomer resin and diene rubber and dispersing the diene rubber in the ionomer resin, wherein pellets of the diene rubber whose surface are sprinkled with ionomer resin powder is mixed with pellets of the ionomer resin, and then a resultant mixture is kneaded at such a temperature at which the diene rubber is crosslinked.
Since the surface of the pellets of the diene rubber is thus sprinkled with the ionomer resin powder whereby the blocking of the diene rubber can be prevented, and the productivity can be improved. Since this prevention of the blocking also permits the ionomer resin and the diene rubber to be uniformly kneaded, it is possible to obtain a golf ball cover material composition constituting the golf ball having superior impact resilience and durability and good appearance.
In order to achieve the second object, the present invention provides a method of manufacturing a golf ball cover material composition constituted by kneading ionomer resin and diene rubber and dispersing the diene rubber in the ionomer resin, wherein the pellets of the mixture of the ionomer resin powder and the diene rubber is mixed with the pellets of the ionomer resin, and then the resultant mixture is kneaded at such a temperature at which the diene rubber is crosslinked.
In such a manner, the mixture of the ionomer resin powder and the diene rubber is pelletized, and these pellets are then mixed with the pellets of the ionomer resin. Thus, since the blocking of the diene rubber can be prevented, the productivity can be enhanced. Since this prevention of the blocking also enables the ionomer resin and the diene rubber to be uniformly kneaded, it is possible to obtain a golf ball cover material composition constituting a golf ball having superior impact resilience and durability and good appearance.
(6) Also, to attain the first object of the invention above, the present invention consists in a golf ball having a core applied with an outer cover which is characterized in that the cover has a Shore D hardness (A) of 63 or below and the golf ball has a compression value (B), the Shore D hardness (A) of the cover having a ratio to the compression value (B) of the golf ball, (A)/(B), within a range of 0.49 to 0.6.
Attributable to the above definition of the hardness of the cover and the ratio (A)/(B), it is possible to attain the first object of the invention. The compression value of the golf ball termed above refers to the same compressive strength (kgf) when the golf ball is deformed to 2.54 mm ({fraction (1/10)} inch) as in the above passage (2). Also, the above termed Shore D hardness refers to a hardness measured according to ASTM D2240 throughout this specification.